1. Field of the Invention
This invention relates to a process of producing polyasparagine having high nitrogen content, and the product produced thereby, using maleic anhydride and ammonia as starting materials, with ammonia as solvent also.
2. Discussion of the Background
Polyaspartates, as very promising biodegradable anionic polymers, have been widely investigated recently, because they have very broad applications. These ionic polymers usually are produced by hydrolysis processes of polysuccinimides with a variety of bases as catalysts. 
Where M is a metal ion, inorganic, or an organic ion.
Various methods for producing polyaspartic acids and polyaspartates have been proposed in the past. Groth et al, U.S. Pat. No. 5,594,077 discloses the production of polyaspartic acid by hydrolysis of polysuccinimide (PSI) to produce a product having pendant carboxylic acid groups, and the salts produced by neutralizing these acid groups. Groth, U.S. Pat. No. 5,493,004, also discloses production of polyaspartic acids having pendant acid groups. This patent discloses a polyasparagine polymer structure, but the polymers disclosed have low molecular weights of about 2700, and which are prepared using aqueous ammonia. The use of aqueous ammonia as disclosed in the patent results in a mixture of ammonolysis and hydrolysis products. The polymers produced are also disclosed as containing PSI groups.
Wagner et al, U.S. Pat. No. 5,530,091, discloses the production of PSI or maleic amide acid by hydrolysis of PSI. Boehmke, U.S. Pat. No. 4,839,461 discloses production of polyaspartic acid and its salts by reaction of maleic anhydride and water, then addition of one mole of ammonia to form the acid ammonium salt of maleic acid.
None of the above described references discloses the preparation of high nitrogen content neutral polyasparagine, in which there are primarily only amide groups pendant on the polymer chain. There is a need for such a high nitrogen content product for use in controlled nitrogen release applications, such as fertilizer.
Accordingly, one object of the present invention is to provide a method for the production of high nitrogen content polyamino acids, particularly polyasparagines.
A further object of the present invention is to provide a method for the production of polyasparagine from polysuccinimide that results in a neutral product substantially free of free acid groups or their salts.
A further object of the present invention is to provide a high nitrogen content neutral polyasparagine useful for controlled release nitrogen applications.
These and other objects of the present invention have been satisfied by the discovery of a method for production of polyasparagine, comprising:
reacting polysuccinimide or a derivative thereof, with liquid ammonia, preferably as both solvent and reactant, at a temperature and pressure sufficient to cause ring opening ammonolysis of the polysuccinimide or derivative thereof, and the polyasparagine produced thereby, as well as its uses in a wide variety of applications.